Warning devices, such as safety marking tape, are widely used in facilities to alert workers to the proximity of an area wherein potentially hazardous chemicals are used or stored. Traditional safety marking tape has on one side diagonal stripes of yellow and black bars for visual indication of a possible hazard and on the other side an adhesive coating to facilitate attachment of the tape to a surface. However, an actual chemical leak or spill may go undetected, as traditional safety tape is not capable of sensing the presence of hazardous chemicals. Thus, it is desirable to have a warning apparatus that is operable to provide visual indication of a hazard area and visual indication that the area has been contaminated by a chemical leak or spill.